UNDERWEIGHT OR OVERWEIGHT?
Your dog is not getting enough to eat if you can easily see its ribs, vertebrae, and pelvic bones, feel no fat on the bones, and possibly notice some loss of muscle mass.  If chronically underfed, adult dogs may experience impaired ability to nurse young and perform work, and increased susceptibility to bacterial infections and parasites; puppies may be stunted in their growth; adult dogs may develop osteoporosis.
Your dog is at an ideal weight if you can easily feel its ribs. The waist should be easily observed behind the ribs when viewed from above.  An abdominal tuck is evident when viewed from the side. 
Your dog is overweight if you cannot feel its ribs, see fat deposits over its back and the base of its tail, discern no waist behind the ribs when viewed from above, and see no abdominal tuck in profile. Obesity occurs in one out of four dogs in western societies.  Its incidence increases with age and is more common in neutered animals. Health risks include diabetes and osteoarthritis.

This information is based on recommendations from the 2006 release of Nutrient Requirements of Dogs and Cats. The report contains useful information for companion animal nutritionists, veterinarians, scientists in industry and academe, regulators, pet owners and anyone with an interest in the health and welfare of these important animals. To order the report, contact the National Academies Press, 500 Fifth Street NW, Washington, DC 20001; (800) 624-6242 or http://www.nap.edu.